1. Field of the Invention
The present invention relates to a process for producing dislocation-free single-crystal rods using the CZ crucible pulling process, in which a seed crystal is immersed in a melt and is pulled out again.
2. The Prior Art
The Czochralski crucible pulling process (CZ crucible pulling process) and the float zone pulling process have become known for the production of highly pure single-crystal rods. In particular, these processes produce monocrystalline silicon rods. During the crucible pulling of crystal rods, in particular of semiconductor rods using the Czochralski method, the monocrystalline or polycrystalline semiconductor fragments which are provided for the production of the melt are generally added to a melting crucible. Then, by heating, the crucible temperature is increased until the contents of the crucible gradually pass into the molten state. Finally, a seed crystal is immersed in contact with the melt and generally, with the crucible and crystal rotating, a monocrystalline crystal rod is pulled from the melt.
This crystal rod is composed of the seed crystal, possibly the seed thin neck, a crystal cone (starting cone), which acts as a transition to the cylindrical crystal rod, and the crystal rod itself. The Czochralski crucible pulling process is extensively explained in, for example, W. Zulehner and D. Huber, Czochralski-Grown Silicon, Crystals 8, Springer Verlag, Berlin-Heidelberg, 1982, and the literature cited therein, taking particular account of what is currently the most important application area, namely the crucible pulling of silicon single crystals.
During the production of single-crystal rods, crystal dislocations frequently occur in the transition region from crystal cone, i.e. the starting cone, to the cylindrical rod part. These dislocations make this crystal rod unusable. These crystal dislocations occur in particular with doped silicon, especially with silicon which is doped with As or Sb. The rod which has been pulled has to be remelted and a renewed, complex attempt at pulling has to be started. However, the number of pulling attempts is limited, for example by the service life of the melting crucible, so that it is no longer possible to pull a defect-free single crystal.
Therefore, it is an object of the present invention to provide a defect-free single-crystal rod and a process for its production.
The above object is achieved according to the invention by a single-crystal rod, obtained using the CZ crucible pulling process, substantially comprising a crystal cone and a cylindrical rod part, wherein the crystal cone has an apex angle of 30xc2x0 to 90xc2x0.
Surprisingly, it has been found that with certain starting cones crystal defects do not occur either in the cone region or in the cylindrical rod part. By way of example, As-doped and Sb-doped crystal rods with a (100) orientation can be obtained without defects with an acute-angled, long cone. If the cone of the crystal is pulled in a specific way (pointed cone), it is found that crystal dislocations no longer occur. The conditions required to achieve this are dependent, inter alia, on the diameter and orientation of the crystal. The crystal cone has an apex preferably from 40 to 60xc2x0.
The length and apex angle of the crystal cone are varied, for example, by rotating the melting crucible and/or the crystal, by increasing or decreasing the temperature of the melt and in particular by means of the crystal-lifting rate.
Consequently, the object is also achieved by a process for producing dislocation-free single-crystal rods using the CZ crucible pulling process, in which a seed crystal is immersed in a melt and is pulled out again, wherein a crystal cone with an apex angle of from 30 to 90xc2x0 is pulled.
Surprisingly, it has been found that, in particular with high dopant quantities, for example, in the case of As-doped and Sb-doped melt material, specific pulling conditions are required for the starting-cone region of the crystal in order to obtain a defect-free crystal rod.
In a further embodiment of the process according to the invention, initially a thin neck and then a crystal cone with an apex angle of 30 to 90xc2x0 is pulled.
The crystal growth, in particular the cone growth, is preferably set by the crystal-lifting rate and by the melting crucible rotation rate and/or the crystal rotation rate. The crystal-lifting rate is preferably from 0.5 to 2 mm/min and particularly preferably from 1 to 1.5 mm/min. The melting crucible rotation rate is preferably from 0 to 20 rpm and particularly preferably from 5 to 15 rpm; the crystal rotation rate is preferably from 10 to 30 rpm and particularly preferably from 15 to 20 rpm.
Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawing which discloses several embodiments of the present invention. It should be understood, however, that the drawing is designed for the purpose of illustration only and not as a definition of the limits of the invention.